1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method of fabricating an LCD device, and particularly, to an In-Plane Switching mode LCD device and a method of fabricating the same.
2. Discussion of the Related Art
Recently, with the development of various portable electronic devices, such as mobile phones, PDAs, notebook computers, etc., a light, thin, small flat panel display device has been in great demand. Research and development are actively conducted for the flat panel display devices, such as an LCD, a PDP (Plasma Display Panel), an FED (Field Emission Display), a VFD (Vacuum Fluorescent Display), etc. Among these devices, the LCD attracts much more attention because of its simple mass-production technique, easy driving system, and implementation of a high picture quality.
There are various display modes for the LCD device according to arrangement of liquid crystal molecules. Currently, a TN (twisted nematic) mode LCD device is being generally utilized because of its easy black and white display, short response time, and low driving voltage. When a voltage is applied to the TN mode LCD device, liquid crystal molecules aligned to be horizontal to a substrate are aligned to be nearly perpendicular to a surface of the substrate. Accordingly, there is a problem in that a viewing angle is narrowed by refractive anisotropy of the liquid crystal molecules in applying of the voltage.
In order to solve this problem, LCD devices of various modes having wide viewing angle characteristics have been proposed. Among those, an In-Plane Switch (IPS) mode LCD device is applied to actual mass-production, and thus is being fabricated. This IPS mode LCD device forms a horizontal electric field that is substantially parallel to a surface of a substrate by forming at least one pair of electrodes arranged parallel in a pixel, so that liquid crystal molecules are aligned along the plane.
FIG. 1 is a schematic view of a structure of the above-mentioned IPS mode LCD device according to the related art. As shown in FIG. 1, a liquid crystal panel 1 has a pixel defined by a gate line 3 and a data line 4 that are disposed along lengthwise and widthwise directions. Although only the (n, m)th pixel is shown in FIG. 1, N (>n) gate lines 3 and M (>m) data lines are disposed in an actual liquid crystal panel, thereby forming N×M pixels over the entire liquid crystal panel 1. A thin film transistor 10 is formed at an intersection of the gate line 3 and the data line 4 in the pixel. The thin film transistor 10 includes a gate electrode 11 to which a scan signal is applied from the gate line 3, a semiconductor layer 12 formed on the gate electrode 11 and activated to form a channel layer when the scan signal is applied thereto, and a source electrode 13 and a drain electrode 14 formed on the semiconductor layer 12, to which an image signal is applied through the data line 4, thereby applying an image signal input from the outside to a liquid crystal layer.
A plurality of common electrodes 5 and a plurality pixel electrodes 7 are arranged substantially parallel to the data line 4 in the pixel. In addition, a common line 16 connected with the common electrode 5 is disposed in the middle of the pixel, and a pixel electrode line 18 connected with the pixel electrode 7 is disposed on the common line 16 and is thus overlapped with the common line 16.
As discussed above, in the related art IPS mode LCD device, the liquid crystal molecules are aligned substantially parallel to the common electrode 5 and the pixel electrode 7. When the thin film transistor 10 is operated and thus a signal is applied to the pixel electrode 7, an horizontal electric field that is substantially parallel to the liquid crystal panel 1 is generated between the common electrode 5 and the pixel electrode 7. The liquid crystal molecules are rotated along the same plane by the horizontal electric field, thereby preventing a gradation inversion due to refractive anisotropy. Here, a black matrix 32 serves to prevent light from being transmitted to an unwanted area, namely, an image non-displayed area.
The related art IPS mode LCD device having such a structure will now be described in more detail with reference to FIGS. 2A and 2B. Here, FIG. 2A is a cross-sectional view taken along I–I′ of FIG. 1 to show a structure of the thin film transistor 10 according to the related art. FIG. 2B is a cross-sectional view taken along 11–11′ of FIG. 1 to show a structure of a pixel according to the related art. In FIG. 2A, the thin film transistor 10 (in FIG. 1) includes the gate electrode 11 formed on a first substrate 20, a gate insulating layer 22 laminated over the entire surface of a first substrate 20, the semiconductor layer 12 formed on the gate insulating layer 22, the source electrode 13 and the drain electrode 14 formed on the semiconductor layer 12. In addition, a passivation layer 24 is formed over the entire surface of the first substrate 20. In FIG. 2B, the plurality of common electrodes 5 are formed on the first substrate 20 in a pixel, and the pixel electrode 7 and the data line 4 are formed on the gate insulating layer 22, so that a horizontal electric field is generated between the common electrode 5 and the pixel electrode 7.
The black matrix 32 and a color filter layer 34 are formed at a second substrate 30. The black matrix 32 serves to prevent light from leaking to an area where the liquid crystal molecules are not operated (namely, undesired area where an image is not displayed), and is usually formed at the thin film transistor 10 area (in FIG. 1) and between pixels (i.e., gate line and data line areas). The color filter layer 34 includes R (Red), B(Blue) and G(Green) colors to implement an actual color. A liquid crystal layer 40 is formed between the first substrate 20 and the second substrate 30, and then the liquid crystal panel 1 is thus completed. In addition, a black resin is usually utilized for the black matrix 32, and the reason thereof will now be described next.
The black matrix 32 made of metal, such as Cr or CrOx, that is usually utilized for the TN mode LCD or the like may form an electric field between the data line 4 and itself by its characteristic low resistance. Such an electric field is a vertical electric field that is formed between the first substrate 20 and the second substrate 30. Meanwhile, another electric field is formed between the pixel electrode 7 on the first substrate 20 and the common electrode 5 on the second substrate 30. As a result, the vertical electric field may not greatly affect the electric field between the pixel electrode 7 and the common electrode 5.
On the contrary, in the IPS mode LCD device, an electric field applied to the liquid crystal layer 40 is a horizontal electric field that is substantially parallel to a surface of the substrate 20 or 30. Accordingly, when the horizontal electric field is formed by the black matrix 32, the vertical electric field affects the horizontal electric field, thereby distorting the horizontal electric field. Such distortion may cause a vertical cross talk on a screen as a main factor of image quality deterioration. Accordingly, in the IPS mode LCD device, a black resin with high resistance is usually utilized as the black matrix 32 to prevent distortion of the horizontal electric field. However, there exist several problems because of disadvantages of the black resin itself.
First, since the resin has a bad anisotropy etching characteristic compared to metal in a photolithography, it is difficult to form the black matrix 32 with a fine pattern to make good resolution. Accordingly, it is difficult to fabricate an IPS mode LCD of high resolution.
Second, since the black matrix 32 has a low light blocking rate compared to Cr or CrOx (because transmittance of light is high), its thickness has to be thick compared to Cr or CrOx in order to completely block light transmitted to the image non-displayed area. Accordingly, a step is generated at the color filter layer 34 and makes it difficult to be flat.
Finally, since the black resin has a bad dispensability compared to the metal, such as Cr or CrOx, a protrusion is easily formed at a surface of the black matrix 32 compared to Cr or CrOx. Thus, defection occurs at the black matrix 32, thereby degrading a yield of the LCD.